This invention relates generally to methods for analyzing data obtained from well logs to determine the hydrocarbon pore volume of a subsurface geological formation. More specifically, this invention relates to an analysis of well log data taken in a reservoir having thinly interbedded sandstone and shale layers.
An estimate of the hydrocarbon pore volume (HPV) of a subsurface geological formation has been obtained by the analysis of well log data obtained from an exploration well penetrating the formation. A well log generally involves the recording of a series of measurements obtained while an instrument traverses the formation through a wellbore. Such measurements are recorded in correlation with uniform depth increments. The well logging instruments produce measurements that represent an average of the measured rock property over a finite length of the wellbore which typically is about three to six feet. The various types of well logging instruments commonly used make measurements based on electrical, acoustic, and nuclear properties of the formation.
In reservoirs where sand and shale are interbedded at bed thicknesses of two feet or less, the vertical resolution limits of the well logging instruments result in HPV estimates being too low. Accordingly, there is a significant risk that economically significant hydrocarbon reserves may be overlooked. Also, if an induction-logging instrument is used to make electrical resistivity measurements in a thinly bedded reservoir, the resistivity measurement becomes less sensitive to the hydrocarbon saturation in the sandstone layers and more sensitive to the proportion of shale. As a consequence, there is less certainty in the estimate of HPV in thinly bedded reservoirs.
Heretofore, two general approaches have been developed to seek more accurate estimates of HPV from well log data obtained in thinly bedded reservoirs. One approach is referred to as “high-resolution,” and the other approach is referred to as “low-resolution.” The high-resolution approach seeks to enhance the vertical resolution of the logging instruments used. The vertical resolution of a logging instrument refers to the thinnest bed in which a true reading can be obtained. The low-resolution approach seeks to determine the relationship between HPV and averaged well log data over a gross formation interval (e.g., 10 feet or more in thickness) so as to obviate having to resolve accurate log data values in individual thin beds.
The essential element of the low-resolution approach is in establishing the relationship within overall interval conductivity of sand layer conductivity and shale layer conductivity. In the simplest case, this relationship can be expressed as: Cint=hsand×Csand+hshale×Cshale, where Cint, Csand, and Cshale are the interval, sand, and shale conductivities, respectively; and hsand and hshale are, respectively, the fractional sand and shale thickness within the interval of interest, with hsand+hshale=1. The interval may be the resolution interval of the resistivity log or may be a larger reservoir interval. Similar equations, well known to those skilled in the art, exist for associated logs such as density, neutron porosity, and gamma ray, which may be used in the estimation of interval hydrocarbon pore volume. Illustrative of the prior art implementations of the low resolution approach are Hagiwara, U.S. Pat. No. 4,739,255, and Frans G. Van den Berg et al., “Sandwich: Log Evaluation In Laminate Shaly Sands,” SPWLA 37th Annual Logging Symposium, Jun. 16-19, 1996.
Also, even though thinly bedded sandstone formations have historically been called shaly sands in the petrophysical literature, standard shaly sand log analysis techniques do not correctly account for the effect of thin bedding on log responses. The standard shaly sand log analysis techniques were developed to address the effects of dispersed clay in sandstone rather than macroscopically interbedded sands and shales. The electrical effects of these two modes of clay or shale distribution are significantly different, and therefore, different log analysis techniques are required to correctly analyze each mode.